The invention relates to liquid crystal display devices using a liquid crystal panel. In particular, the invention is concerned with a reflection-type liquid crystal display device utilizing external light for effecting display without a light source built therein, and a transflective liquid crystal display device having an auxiliary light source built therein to be lit in a dark application environment.
A display capacity of liquid crystal display devices has recently been on the steady increase. There are two types of liquid crystal display devices in constitution, that is, a passive-matrix type wherein display electrodes for liquid crystal pixels are directly connected to signal electrodes installed on a first substrate thereof, and an active-matrix type having switching elements installed between signal electrodes and display electrodes.
Further, the active-matrix type liquid crystal display device has a constitution wherein opposed electrodes are installed opposite to the display electrodes provided on the first substrate with liquid crystals interposed therebetween, and a plurality of the signal electrodes and a plurality of the opposed electrodes are disposed in a matrix fashion such that given signals from an external circuit are applied to respective data electrodes connected to the signal electrodes and the opposed electrodes.
In the case of applying multiplex driving to a liquid crystal display device of a simple matrix constitution (the passive-matrix type), deterioration in contrast or response time occurs as the multiplexing reaches a higher time-division rate, so that it becomes difficult to obtain sufficient contrast when scanning lines in the order of two hundred.
Accordingly, a liquid crystal display panel of the active-matrix type wherein individual pixels are provided with a switching element has been adopted in order to remove such a drawback.
For the switching element in the liquid crystal display panel of the active-matrix type, there are in use a three-terminal type switching element using a thin-film transistor, and a two-terminal type switching element using a nonlinear resistance element. Of these two switching elements, the two-terminal type switching element is regarded as superior in respect of having a simpler construction and an adaptability to fabricate by a relatively low temperature process.
For the two-terminal type switching element, there have been developed a diode-type, a varistor-type, a thin-film-diode (TFD) type, and so forth.
Among these types, the TFD type is characterized especially by simple construction and in addition, by a short-time fabrication process.
Further, since the liquid crystal display device is not a self-light-emitting type display device, display is effected by utilizing an external light source, thereby causing variation in external light due to the optical property of liquid crystals.
Accordingly, the liquid crystal display device is broadly classified into two kinds in terms of relative positions of a viewer, the liquid crystal display device, and the light source. First, there is one wherein the light source (a main light source) and the viewer are on the same side relative to the liquid crystal display device, the so-called reflection-type liquid crystal display device. Second, there is one wherein the viewer, the liquid crystal display device and the light source (the main light source) are disposed in that order, the so-called transmission-type liquid crystal display device.
In the case where an object is to achieve low power consumption, which is the merit of a liquid crystal display device, the reflection-type liquid crystal display device taking advantage of the light source located in the neighborhood thereof without the light source installed therein is more effective.
There is also available a transflective liquid crystal display device functioning as the reflection-type liquid crystal display device taking advantage of the external light source (the main light source) when an application environment thereof is bright while functioning as the transmission-type liquid crystal display device by lighting up an auxiliary light source built therein when the application environment is dark.
Since the transflective liquid crystal display device is employed basically as the reflection-type liquid crystal display device, its power consumption can be lowered in comparison with that for the transmission-type liquid crystal display device. It is for this reason that the reflection-type liquid crystal display device or the transflective liquid crystal display device is a very important display device for application to portable information equipment.
A conventional reflection-type liquid crystal display device having the two-terminal type switching elements as switching elements installed between signal electrodes and display electrodes is described hereinafter with reference to the accompanying drawings.
FIG. 26 is an enlarged plan view showing an electrode constitution at a pixel of the conventional reflection liquid crystal display device using the two-terminal type switching element. FIG. 27 is a partial sectional view of the conventional reflection-type liquid crystal display device, taken along line Axe2x80x94A in FIG. 26.
As shown in FIG. 27, with this reflection-type liquid crystal display device, a pair of a first substrate 1 and a second substrate 2, made up of a transparent glass substrate, are disposed opposite to each other with a given gap provided therebetween. On the surface of the second substrate 2, a signal electrode 3, made up of a tantalum (Ta) film, and a lower electrode 4, formed integrally with the signal electrode 3 so as to be projected sideways from the side thereof (refer to FIG. 26) are provided. On the surface of the signal electrode 3 and the lower electrode 4, a nonlinear resistance layer 5 made up of a tantalum oxide (Ta2O5) film is provided.
Further, an upper electrode 6 and a display electrode 9 formed integrally with the upper electrode 6 as shown in FIG. 26, made up of an indium tin oxide (ITO) film which is a transparent and electrically conductive film, are provided so as to overlap the nonlinear resistance layer 5 provided on top of the lower electrode 4. The upper electrode 6, the nonlinear resistance layer 5, and the lower electrode 4 constitute a two-terminal switching element 7.
On the inner face of the first substrate 1, facing the second substrate 2, there is disposed an opposed electrodes 12, formed in stripes and made up of an indium tin oxide (ITO) film which is a transparent and electrically conductive film, enabling it to face the display electrode 9. Further, a data electrode (not shown) for applying signals from an external circuit is connected to the respective opposed electrodes 12.
Further, on the inner faces of the first substrate 1 and the second substrate 2, facing each other, there are provided alignment layers 15A, 15B, respectively, functioning as layers for alignment treatment wherein molecules of liquid crystals 16 sealed in-between the inner faces are aligned in a regular fashion.
The first substrate 1 and the second substrate 2 are disposed so as to face each other across a given gap formed by spacers (not shown), and the liquid crystals 16 are sealed in the gap.
Further, polarizing films 21A, 21B are disposed on the outer faces of the first substrate 1 and the second substrate 2, respectively, and a reflector 25 is disposed on a side of either of the polarizing films 21A, 21B, opposite from the liquid crystal 16 (in an example shown in FIG. 27, on the outer face of the second substrate 2). There is a case where the polarizing films 21A, 21B are required and a case where the polarizing films 21A, 21B are not required, depending on the type of the display mode of the liquid crystal display device, for example, phase-transition type guest-host (p-GH) mode, twisted-nematic (TN) mode, and so forth.
Since no light is emitted by the liquid crystal display device itself, a voltage at driving waveforms is applied from an external circuit to the signal electrodes 3 and the data electrodes not shown (connected to the opposed electrodes). By applying the voltage via the switching elements 7 to the liquid crystal 16 in regions between the display electrodes 9 and the opposed electrodes 12, thereby causing changes in the optical property of the liquid crystal 16 to occur, and in addition, by taking advantage of the reflection characteristics of the reflector 25 and external light 31, a display of desired images is effected.
However, with the conventional liquid crystal display device described above, images displayed had a good contrast ratio but were found lacking in brightness, especially, whiteness, so that its display performance was less than satisfactory. In the case of using color filters, brightness further declined.
Further, in the case where projections and depressions are provided on the surface of the reflector, control of the surface shape thereof and enhancement of reflectance are needed, thus involving complex work in forming the reflector. Furthermore, in the case of employing a white diffusion film having a specific polarizability, it becomes necessary to match the direction of light diffusion having a directivity dependent on the surface shape with the direction of polarization, and consequently, a white diffusion film having dependence on the direction of polarization of the liquid crystal display device needs to be prepared, so that general versatility is impaired.
Also, with the reflection-type liquid crystal display device wherein losses in light quantity occur due to installation of the polarizing films, it becomes necessary to enhance brightness by utilizing the light diffusion film in combined use of the polarizing films and the reflector such that losses in light quantity can be prevented as much as possible.
Furthermore, in the case where the liquid crystal display device is provided with an auxiliary light source, it is necessary to enhance brightness by utilizing the white diffusion film in combined use of the auxiliary light source and the reflector.
The invention has been developed to solve the problems described above, and an object of the invention is to provide a reflection-type liquid crystal display device capable of effecting bright and whitish display.
In order to achieve the object described above, the liquid crystal display device according to the invention comprises a first substrate made of a transparent material, provided with signal electrodes or display electrodes, formed on one face thereof, a second substrate made of a transparent material, provided with opposed electrodes formed thereon, and liquid crystals sealed in-between the first substrate and the second substrate, disposed oppositely to each other with a predetermined gap interposed therebetween such that each of the signal electrodes or the display electrodes faceseach of the opposed electrodes so as to form a pixel,
wherein a white diffusing film and a reflector are disposed in this order from the side of the first substrate on the outer face of the second substrate, and the white diffusing film has characteristics of allowing circularly polarized light to pass therethrough substantially as the circularly polarized light, and having a substantially equal transmittance for light components at respective wavelengths in the wavelength range of visible light.
Otherwise, a polarizing film and a white diffusing film may be disposed in this order from the visible side on top of the first substrate, and a reflector may be provided on top of the second substrate, or a polarizing film and a reflector may be provided in this order thereon.
The opposed electrodes formed on the second substrate may concurrently serve as the reflector.
Further, a polarizing film may be disposed on the visible side of the first substrate while a white diffusing film, a polarizing film and a reflector may be disposed in that order on the outer face of the second substrate. Or the order in which the white diffusing film and the polarizing film are disposed may be reversed. Further, another white diffusing film may be disposed between the first substrate and the polarizing film.
The reflector may be made of a transflective reflector having the characteristic of a substantially equal transmittance for light components at respective wavelengths in the wavelength range of visible light.
Otherwise, the transflective reflector may be made of a reflection-type polarizing film wherein one of the optic axes thereof is the transmission axis and the other, orthogonal to the transmission axis, is the reflection axis.
Further, the transflective reflector may be made of a holographic reflection film wherein regions having different refractive indexes are spatially distributed.
The liquid crystal may be a liquid crystal containing a dichroic pigment.
A color printed layer and a white diffusing film may be disposed in an optional order on the second substrate while a reflector may be disposed on a side of the color printed layer and the white diffusing film, opposite the visible side. In addition, the white diffusing film and the color printed layer may have the characteristics of allowing circularly polarized light to pass therethrough substantially as the circularly polarized light, respectively, the white diffusing film may have the characteristic of having a substantially equal transmittance for light components at respective wavelengths in the wavelength range of visible light, and the color printed layer may have a transmittance having wavelength dependency.
Further, a white diffusing film, a reflector, and a light absorption layer may be disposed in that order from the visible side of the second substrate, and the reflector may be a reflection-type polarizing film, and the light absorption layer may have a reflectance lower than that of at least the white diffusing film.
A white diffusing film and a color printed layer may be disposed in an optional order on the second substrate while a reflector and a light absorption layer may be disposed in this order on a side of the white diffusing film and the color printed layer, opposite the visible side. Further, the reflector may be a reflection-type polarizing film, and a reflectance of the color printed layer towards the side of the second substrate may be rendered smaller than that of the light absorption layer towards the side of the second substrate.
The color printed layer or the light absorption layer may be composed of a plurality of portions, each having a transmittance with a wavelength characteristic in the wavelength range of visible light.
It is preferable that the white diffusing film has a transmittance of at least 70%.
The white diffusing film may be made of a complex substance comprised of resin beads and a synthetic resin having a refractive index differing from that of the resin beads, and may have a light-scattering characteristic due to the difference in refractive indexes therebetween.
The white diffusing film may be a white diffusing film with a plurality of projections and depressions formed on the surface thereof, causing a portion of light incident on the surface thereof to undergo diffuse reflection and remaining portions of the light to be transmitted therethrough, and the projections and depressions formed on the surface may be formed in a shape approximating to a quadratic curve.
The white diffusing film may have regions thereof, corresponding to respective pixels, that have diffusibility differing from that of regions around the respective pixels.
Further, a white diffusing film and a reflector may be disposed in this order from the side toward the first substrate on the outer face of the second substrate, and the white diffusing film may allow circularly polarized light to pass therethrough substantially as the circularly polarized light while respective pixels may be provided with color filters.
The white diffusing film may be made up of a diffusing-type liquid crystal layer for diffusing light.
In such a case, the white diffusing film preferably comprises two transparent substrates, provided with an electrode formed on the inner faces thereof, facing each other, respectively, and a mixed liquid crystal layer comprised of transparent solids and liquid crystal, that is sandwiched between the two transparent substrates, so that a degree of light scattering caused by the mixed liquid crystal layer can be rendered variable according to voltage by applying a voltage between the respective electrodes described in the foregoing.
The invention also provides a liquid crystal display device wherein an auxiliary light source is provided on a side of the second substrate, opposite the visible side, in the case of the liquid crystal display devices described in the foregoing, provided with a transflective reflector as the reflector.
In the liquid crystal display device according to the invention, the white diffusing film is provided on a face of the first substrate or the second substrate constituting the liquid crystal display panel, opposite a side of the first substrate or the second substrate, adjacent to the liquid crystal, and the white diffusing film reflects a portion of incident light but allows most of the remaining portions to be transmitted therethrough. Further, light rays gain directivity when transmitted through the white diffusing film, then alter the direction of incidence from the light source, are partly diffused, and gain additional whiteness before sent out again to the side of a viewer, so that display is rendered brighter and particularly, whiteness of the display is enhanced.